/* //正向肯定 就往右看，必须得跟着什么pattern
let str = 'a1b2c3';
let regexp = /[a-z](?=\d)/;
let result = regexp.exec(str);
console.log(result);
 */

/* //正向否定，往右看，后面不能跟着什么
let str = 'a1b2c3d';
let regexp = /[a-z](?!\d)/;
let result = regexp.exec(str);
console.log(result); */

/* //反向肯定
let str = 'a1b2c3d';
let regexp = /(?<=\d)[a-z]/g;
let result = regexp.exec(str);
console.log(result);//b
result = regexp.exec(str);
console.log(result);//c
result = regexp.exec(str);
console.log(result);//d */

/* //反向否定
let str = 'a1b2c3d';
let regexp = /(?<!\d)[a-z]/g;
let result = regexp.exec(str);//a
console.log(result);
result = regexp.exec(str);//a
console.log(result); */


/* let str = 'a1b2c3';
let regexp = /[a-z](?=\d)\d/;
let result = regexp.exec(str);
console.log(result);//a1 */


